Scotland Yard is using controversial live facial recognition technology near Oxford Circus.
The van-mounted cameras were spotted at the busy central London location on Thursday morning.
It is believed to be the second time the Metropolitan Police have used the surveillance which has been criticised by privacy campaigners.
A photograph of a dark blue van, next to a sign reading "Police live facial recognition in use", taken near Oxford Circus was later posted on Twitter.
It's alarming to see biometric mass surveillance being rolled out in London. Never before have citizens been subjected to identity checks without suspicion, let alone on a mass scale. > We're appalled that Sadiq Khan has approved such useless, dangerous and authoritarian surveillance technology for London.
Trials of the technology saw cameras used in locations including the Westfield shopping centre in Stratford and London's West End, while operational use began earlier this month at the Stratford Centre.
People scanned by the cameras are checked against "watchlists" - said to contain suspects wanted by police and the courts - and approached by officers if there is a match.
The Met claims that the technology has a very low failure rate, with the system only creating a false alert one in every 1,000 times.
However, using a different metric, last year research from the University of Essex said the tech only achieved eight correct matches out of 42, across six trials it evaluated.
A spokesman for the Mayor of London said Mr Khan has asked for reassurance that police have met conditions set out in a report on the ethical use of the technology by an independent panel.
New technology has a role in keeping Londoners safe, but it's equally important that the Met Police are proportionate in the way it is deployed and are transparent about where and when it is used in order to retain the trust of all Londoners. City Hall and the ethics panel will continue to monitor the use of facial recognition technology as part of their role in holding the Met to account.